Watts and voltamps

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mojoman

Member
Location
mass
Havent had to do many load cal in the passed 15 yrs. Im a resi guy. problem with Nstar. Are Voltamps and watts the same..... for a load calulation?
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
No, in general they are not. But for many of the things we deal with, they are the same. For motor loads, the VA value will be higher than the W value. For a range, the two values will be the same. Are you looking at anything in particular?

Welcome to the forum.
 

mojoman

Member
Location
mass
At the risk of sounding stupid, I have to do load cals for Nstar and everything in kva. It's just for 3 units. I have all the watt ratings.
 

templdl

Senior Member
Location
Wisconsin
Now. questions to clarify the OP: What is meant by "units?' Are they individual devices such as ovens, ranges, or? Again, just a guess at this point, if what you have is watts it may be heating watts which it may very well be that it will be the same as VA.
 

Jraef

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Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
At the risk of sounding stupid, I just need to do load cals for Nstar. Everything is rated in watts and they require kva

Watts = VA x PF (Power Factor), so of all you know is watts, then VA = W/PF

If the load is resistive in nature, i.e. incansecent lamps (lol) or resistive heaters, then the PF is 1.0 so W = VA

If the load is inductive in nature, like motors, electronic power supplies etc., then you need to know the Power Factor to determine VA, no way around it. You can make a SWAG at it by ASSuming the PF is .80, but that's all it is, a SWAG. If you want an extra conservative VA value from only knowing watts, you can use .70 PF. It's very rare to see loads with lower than a .70 PF.

So for example lets say you have a 4kW load and you want to know the kVA. If you don't know the PF of the load and want to be conservative, you take 4kW/.7 = 5.71kVA. it will likely be less, but this may be the only way to be safe at guessing.
 

mojoman

Member
Location
mass
Nstar is the power company. I gave them load calculations using NEC calculations but they want everything in KVA.

Lighting load
Appliances
Micro
Laundry
Heat
A/C
Misc load

They do not change the demand factor, they just want to know what is being supplied and its full load. Its a pain.
 
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templdl

Senior Member
Location
Wisconsin
Nstar is the power company. I gave them load calculations using NEC calculations but they want everything in KVA.

Lighting load
Appliances
Micro
Laundry
Heat
A/C
Misc load

They do not change the demand factor, they just want to know what is being supplied and its full load. Its a pain.

This is a bit more than what you asked in your OP. Then Jraef has answered your question.
 
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